the thickness at which ice no longer grows because it is so thick that heat from the ocean can no longer be conducted through the ice; it may take several years of growth and melt for ice to reach an equilibrium thickness.
the northern parallel of maximum solar declination, approximately 23° 27′ N latitude; the farthest northern latitude at which the sun can appear directly overhead, which occurs on the day of the summer solstice.
the southern parallel of maximum solar declination, approximately 23° 27′ S latitude; the farthest southern latitude at which the sun can appear directly overhead, which occurs on the day of the summer solstice.
the boundary layer between the troposphere and stratosphere, where an abrupt change in temperature lapse rate usually occurs; it is defined as the lowest level at which the lapse rate decreases to 2 degrees Celsius per kilometer or less, provided that the average lapse rate between this level and all higher levels within 2 kilometers does not exceed 2 degrees Celsius per kilometer; occasionally, a second tropopause may be found if the lapse rate above the first tropopause exceeds 3 degrees Celsius per kilometer.
lower part of the atmosphere, extending from the surface up to a height varying from about 7 to 9 kilometers (4.3 to 5.6 miles) at polar regions to approximately 17 kilometers (10.6 miles) in tropics; characterized by decreasing temperature with height, appreciable vertical wind motion, appreciable water vapor content, and weather.